Typescript：如何提取 const 对象的值类型并将它们用作新类型中的键？

Question

我想获取一个常量对象，比如{key1: 'value1', key2: value2'} as const，并将其值转换为键，并在类型为{value1: number; value2: number} 的新对象中使用它。我想在函数中执行此操作，并且希望正确键入函数。

下面是我尝试开始工作的代码。当我尝试将Partial<Blah> 类型的变量转换为Blah 时，就会发生错误。有人可以解释为什么会发生这种情况，以及是否有办法做我想做的事情？谢谢！

type BaseObject = Record<string, string>;


type ValuesAsKeys<T extends BaseObject> = {
    [K in keyof T as T[K]]: number;
};

function useValuesAsKeys<T extends BaseObject>(arg: T): ValuesAsKeys<T> {
  const x: Partial<ValuesAsKeys<T>> = {};
  for (const value of Object.values(arg) as Array<T[keyof T]>) {
    x[value] = 1;
  }

  // This line doesn't work
  return x as ValuesAsKeys<T>;
}

// Example use case
const obj = {key1: 'value1', key2: 'value2'} as const;
const result = useValuesAsKeys(obj); // Type of result is {value1: number; value2: number}


// Everything below here works
// Only including this because I'm contrasting this code with the code above.

type KeysAsKeys<T extends BaseObject> = {
    [K in keyof T]: number;
};

function useKeysAsKeys<T extends BaseObject>(arg: T): KeysAsKeys<T> {
    const x: Partial<KeysAsKeys<T>> = {};
  for (const value of Object.values(arg) as Array<T[keyof T]>) {
    x[value] = 1;
  }
  return x as KeysAsKeys<T>;
}

Answer 1

我仍然不确定为什么在这种情况下将Partial<Blah> 转换为Blah 不起作用，但是这段代码有效并且对我来说足够安全：

function useValuesAsKeys<T extends BaseObject>(arg: T): ValuesAsKeys<T> {
  const x = {} as ValuesAsKeys<T>;
  for (const value of Object.values(arg) as Array<T[keyof T]>) {
    x[value] = 1;
  }
  return x;
}

Answer 2

带有reduce 的显式返回类型几乎总是不起作用，因为initial 的值每次迭代都会改变。

为了让它工作，最好重载你的函数：

type BaseObject = Record<string, string>;


type ValuesAsKeys<T extends BaseObject> = {
  [K in keyof T as T[K]]: number;
};

type Values<T> = T[keyof T]

function useValuesAsKeys<T extends BaseObject>(arg: T): ValuesAsKeys<T>
function useValuesAsKeys<T extends BaseObject>(arg: T) {
  return (Object.values(arg) as Array<Values<T>>).reduce((acc, elem) => ({
    ...acc,
    [elem]: 1
  }), {})
}

// Example use case
const obj = { key1: 'value1', key2: 'value2' } as const;

const result = useValuesAsKeys(obj); // Type of result is {value1: number; value2: number}
result.value1 //ok

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